This invention relates to wobble pistons, and more particularly to a wobble piston and cylinder arrangement that minimizes clearance volume.
Wobble pistons are used in oilless air compressors and vacuum pumps. They include a peripheral seal on the piston head that engages the cylinder bore. The piston head and its connecting rod are fixed to each other, and the connecting rod is mounted on an eccentric on a shaft. As the eccentric is turned by the shaft, the wobble piston will be moved in and out and will "wobble" from side to side. Examples of wobble pistons are found in U.S. Pat. No. 3,961,868 issued Jun. 8, 1976, for "Air Compressor" and U.S. Pat. No. 5,092,224 issued Mar. 3, 1992, for "Conical Rod Piston".
A valve plate typically closes the open top end of the cylinder bore. At the top dead center position of the piston within the cylinder, some clearance must be provided between the top of the piston head and the valve plate, even if the valve plate is flat. The amount of clearance necessary depends upon the accumulated tolerances. However, as the wobble piston leaves top dead center, the tilting motion of the connecting rod results in one side of the piston head traveling higher than the center of the head. Typically, points along the diameter of the piston head will reach their maximum heights within the first 25.degree. of motion away from top dead center. If the top of the piston head is flat and the valve plate is flat, the length of the connecting rod must be adjusted for this worst tip clearance condition. The result is that the clearance volume at top dead center must be increased. This results in lower pressures within the cylinder and lower efficiencies.
One solution to the clearance volume problem has been to use symmetrical bevels at the top edges of the piston head to allow a longer rod length and resulting in a lower clearance volume. The small amount of volume added by the bevels at top dead center condition, is outweighed by the volume reduction from a longer connecting rod. Another solution has been to use a dome-shaped piston head.
The present invention is directed to further optimizing the clearance volume without the need for tighter tolerances.